The polypeptide growth factor hepatocyte growth factor/scatter factor (HGF/SF) (Gherardi et al., 1989; Miyazawa et al., 1989; Nakamura et al., 1989; Stoker et al., 1987) and its receptor MET, the product of the c-MET protoncogene (Bottaro et al., 1991), play essential roles in the development of epithelial organs such as the placenta and liver (Schmidt et al., 1995; Uehara et al., 1995) and in the migration of myogenic precursor cells (Bladt et al., 1995) and motor neurons (Caton et al., 2000; Ebens et al., 1996).
HGF/SF and MET are also involved in the spreading of a variety of epithelial tumours as a result of MET chromosomal rearrangements (Yu et al., 2000), somatic and/or germline mutations in the MET kinase (Schmidt et al., 1997) or, more often, over expression in tumour cells of an unrearranged and unmutated MET gene (reviewed in Jeffers et al., 1996).
HGF/SF has a unique domain structure that resembles that of the blood proteinase precursor plasminogen and consists of six domains: an N-terminal (N) domain, homologous to plasminogen activation peptide, four copies of the kringle (K) domain and a catalytically inactive serine proteinase domain (Donate et al., 1994). Two products of alternative splicing of the primary HGF/SF transcript encode NK1, a fragment containing the N and the first K domain, K1, (Cioce et al., 1996), and NK2, a fragment containing the N, K1 and second kringle, K2, domains (Chan et al., 1991; Hartmann et al., 1992; Miyazawa et al., 1991). Both NK1 (Lokker and Godowski, 1993) and NK2 (Chan et al., 1991) were initially characterized as MET antagonists, although experiments in transgenic mice have subsequently indicated that NK1 behaves in vivo as a bona fide receptor agonist (Jakubczak et al., 1998).
There is an important difference in the mechanism of receptor binding and activation by HGF/SF and NK1. HGF/SF is fully active in cells lacking heparan sulphate, while NK1 is only active in cells that display heparan sulphate or in the presence of soluble heparin (Schwall et al., 1996). Thus NK1, but not HGF/SF, resembles FGF (Rapraeger et al., 1991; Yayon et al., 1991) in terms of a requirement for heparan sulphate for receptor binding and/or activation.
Early domain deletion experiments indicated that the N domain is important for heparin binding (Mizuno et al., 1994) and site-directed mutagenesis identified residues in this domain essential for binding (Hartmann et al., 1998). Thus reverse-charge mutation of R73 and R76 decreased the affinity of HGF/SF for heparin by more than 50 fold (Hartmann et al., 1998). A role for several other positively-charged residues, such as K58, K60 and K62, was suggested from the solution structure of the N domain, as these residues are clustered in close proximity of R73 and R76 (Zhou et al., 1998), and recent NMR experiments have provided experimental support for an involvement of K60, K62, R73, R76, R78 and several other residues in heparin binding to the N domain (Zhou et al., 1999).
Despite this progress, the mechanism through which heparin and heparan sulphate confer agonistic activity to NK1 remains incompletely understood. NK1 crystallizes as a dimer in the absence of heparin (Chirgadze et al., 1999; Ultsch et al., 1998), and the features of this dimer suggested that it could represent the biologically active form of NK1 (Chirgadze et al., 1999). No experimental evidence, however, supports this interpretation as yet.